Most high performance tennis racquet frames today are made of resin materials reinforced with fibers, particularly carbon fibers which are used to make the so called graphite frame.
The standard method of producing composite material racquet frames is by using thermoset prepregs. These come in the form of unidirectional sheets, woven fabrics, or braided tubes.
In a conventional method, fiber-impregnated resin tennis racquet frames are made starting with sheets of a prepreg material composed of aligned carbon fibers impregnated with uncured thermoset resin, e.g. a B-stage epoxy resin. An uncured thermoset resin is neither liquid nor solid, and creates a prepreg which is very drapable, soft and formable. It is also very tacky so that plies stick to each other and can be rolled up and packed in the press.
According to this method, the sheets are cut at specific angles and rolled into the shape of a tube. Successive layers are oriented at alternating angles, in order to impart directional rigidity and strength to the finished frame. Usually, the tube is formed on a mandrel over an inflatable bladder or hand rolled. The mandrel is then withdrawn and the tube, still containing the bladder, is packed into a mold in the shape of a frame. In the mold, the bladder is inflated forcing the prepreg tube to conform to the shape of the mold, and the prepreg is heat cured and hardened.
The foregoing method has the advantage of being able to produce high quality racquet frames, but has the disadvantage of requiring a significant amount of labor in forming the prepreg and packing it in the mold. Also, when using this method the prepreg tubes need to be stored in a cool environment. The cured frame also requires significant work in hand finishing when removed from the mold.
According to another method, reinforcing fibers coated with B-stage epoxy are braided into tubes. A bladder is inserted into the tube, and the tubes are internally pressurized and cured in a mold in a manner similar to the process described above for prepreg sheets. The epoxy-coated fibers, however, are tacky, and it thus may be difficult to form and work with the braided tubes. As a result, it may be desirable to limit the choice of materials to an epoxy which is not tacky in the B-stage.
It has also been proposed to make tennis racquets using a thermoplastic, rather than a thermosetting, resin. However, the above-described process cannot, as a practical matter, be used with thermoplastics. In contrast to uncured epoxy prepregs, which at room temperature are drapable, a thermoplastic prepreg would be very hard. In order to use the above process it would thus be necessary to heat the resin to a very high temperature first to form the tube, and again in order to pack the tube in the mold, making it difficult for workers to handle in racquet forming operations. Also, a thermoplastic prepreg would possess no tack, making it difficult to form a tubular layup using multiple layers. Additionally, thermoplastic materials have a relatively small window near the melting point before they start to flow. Even assuming that the prepreg could be heated to the softening point without melting the resin, it would be difficult to maintain constant temperature during processing.
There have been several proposals made, and several tennis racquets introduced to the market, which are made of injection molded, fiber-reinforced thermoplastics. However, these do not employ the same process used to make thermoset racquets, and do not have the same fiber structure. Instead, injection molded thermoplastic racquets are formed using a mixture of resin and short length fibers, which is injected into a racquet mold. The fibers are disbursed through the resin in a random orientation to produce a material with isotropic properties. This has the advantage of simplifying the racquet-forming process, in that the number of manual steps is reduced. However, injection molding processes possess the major limitation that, because the fibers pass through an injection nozzle, they cannot exceed about 1/2 inch. Inherently then, the reinforcing fibers are much shorter than in a prepreg thermoset resin process, and do not produce the same strength and overall racquet stiffness as in the case of the longer fibers present in a thermoset racquet. Moreover, known thermoplastic processes have the disadvantage that the fiber orientation cannot be controlled.
U.S. Pat. No. 4,643,857 recognizes the fiber length limitation with injection molded thermoplastic racquets, and proposes making a racquet using an extrusion process so that the fiber length can be increased. However, according to the '857 patent, it achieves fiber lengths of only 5 or 6 mm. Such fibers are considerably shorter than the reinforcement fibers used in the thermoset process described above.
It would be desirable to produce a thermoplastic frame which contains reinforcing fibers of much greater length, and preferably of a length comparable to those in known thermoset processes. It would also be desirable to produce a thermoplastic frame in which the orientation of the fibers can be controlled to produce predetermined angular stiffnesses.